Translate area mouse coordinates to the the Node Editor's blackboard coordinates?
this'll solve it for you:
def invoke(self, context, event):
    region = context.region.view2d  
    ui_scale = context.preferences.system.ui_scale     
    x, y = region.region_to_view(event.mouse_region_x, event.mouse_region_y)
    self.x,self.y = x /ui_scale,y/ui_scale
    return self.execute(context)


Then when you want to use it:
def execute(self,context):
    new_node.location.x = self.x
    new_node.location.y = self.y

    return {'FINISHED'}